code: def test_sobel_pure():
      f = np.random.random((64, 128))
      f2 = f.copy()
      _ = mh.sobel(f)
      assert np.all(f == f2)
  
file_path: mahotas/mahotas/tests/test_edge.py
imports: from mahotas.edge import sobel
  import pytest
  import mahotas
  import numpy
label_desc: Use image node_id 5 for calling a global function with example usage: test_sobel_pure() without return types